Problem: Simplify the following expression and state the condition under which the simplification is valid. $y = \dfrac{n^2 - 49}{n - 7}$
Explanation: First factor the polynomial in the numerator. The numerator is in the form ${a^2} - {b^2}$ , which is a difference of two squares so we can factor it as $({a} + {b})({a} - {b})$ $ a = n$ $ b = \sqrt{49} = -7$ So we can rewrite the expression as: $y = \dfrac{({n} {-7})({n} + {7})} {n - 7} $ We can divide the numerator and denominator by $(n - 7)$ on condition that $n \neq 7$ Therefore $y = n + 7; n \neq 7$